When development teams submit their bids next month to build what could be San Francisco's tallest skyscraper, one key detail will be missing: the actual height of the building.

The tower proposed for First and Mission streets is part of larger efforts to build a new terminal for buses and commuter trains on the same block, and planning officials a year ago suggested raising the heights around the new station -- perhaps above 1,000 feet.

But the Planning Department has yet to begin studies on whether such heights make environmental sense at that location. So for now the zoning remains at 550 feet -- and the teams seeking to buy or lease the land from the Transbay Joint Powers Authority don't know exactly how large a tower they might be able to build.

Despite this, officials say the competition is on track -- and the loose ends shouldn't cause the project or competition to unwind.

"I don't think it affects us in a bad way at all," said Maria Ayerdi, executive director of the Transbay Joint Powers Authority. "We need to have a design team on board by April or May of 2008, and we're on schedule."

The authority was created in 2001 to replace the existing Transbay Terminal with a new transit center that would be the destination of buses from throughout the Bay Area and passenger trains from the South Bay and perhaps even Southern California.

The overall project cost is estimated at $3.4 billion, $983 million of which would fund the initial phase that includes tearing down the existing terminal and erecting its replacement.

The tower is a critical piece of the package, since authority officials estimate a developer would pay at least $360 million to control the site at the southeast corner of First and Mission streets where a high-rise would be allowed.

To whet interest, the authority in November launched an international competition for a "design and development team" that would not only build a tower but also design the new terminal.

Five teams responded; four were invited to submit full proposals that are due July 10. Last month one team -- made up of Boston Properties and renowned Spanish architect Santiago Calatrava -- pulled out of the running.

The remaining three are formidable. Forest City Enterprises and McFarlane Properties are working with English architect Richard Rogers and the San Francisco firm SMWM. The development firm Hines is with Pelli Clark Pelli Architects of New York, and Rockefeller Group has paired with the San Francisco office of Skidmore Owings Merrill. During a review in late May where the teams went through technical issues with authority staff, Rogers and Cesar Pelli both flew to San Francisco for the presentations.

A seven-member jury composed largely of design professionals will choose the winning team based on factors such as the tower's demonstration of "green design, seismic and structural innovation." The competition rules also state that the jury "will focus on the timing and amount of revenue" to the authority.

The reality of the situation, however, makes it likely that teams will propose a range of economic offers since they're unable to say definitely how large and tall their tower might be.

For a time it seemed the sky was the limit: Planners in May 2006 proposed a cluster of towers starting at 850 feet near First and Mission. That's the height of the Transamerica Pyramid, now the city's tallest structure.

Additional height would boost the value of Transbay land. Also, by allowing two or three other extra-tall towers, the additional property taxes could be steered toward the project.

Aesthetics were a factor as well: "We have not paid a lot of attention to the drama of our skyline," Planning Director Dean Macris fretted at the time.

But Macris -- who was on vacation last week and unavailable for comment -- stressed that zoning changes would not be done without first studying the impact of such towers on shadows, winds, traffic and other elements of the downtown environment.

The assumption was that such a study would be started by the fall. Instead, it was December before the Planning Department issued a request for consultants to provide technical assistance on the zoning review. When both respondents also were on Transbay teams, complaints from competition rivals led the department to stop the process.

In May the department started again -- seeking a consultant to review tower prototypes from a seismic and safety perspective as well as urban design. The firm must also deliver results "as expeditiously as possible."

Submissions are due on June 18. The department wants to release a draft set of zoning proposals next spring.

In the meantime, the competition teams were directed in March to expect a maximum height of between about 800 and 1,200 feet, while striving for a design "as slender as possible in all aspects."

At the Transbay Authority, Ayerdi voiced confidence that the submissions in July will be strong and that a team will be selected in September, with contract negotiations then beginning in earnest.

"The contract we negotiate will be complex," Ayerdi said. "By the time all the details get ironed out, the Planning Department will have the information in place. I'm staying positive."